Superheat and reheat control



Patented Dec. 22, 1953 aessesw p 'SUPEBHEAT AND REHEAT CONTROL Wilbur .H.-Armacost, Scarsdale, N. Y.,.assignor to Combustion Engineering, Inc.,' a corporation 7 of Delaware 7 Application September 17, 1948 Serial No. 59,699

The; present invention relatesto steam gen erators and particularly to an improved method of regulating the temperature of superheated steam in a generator equipped with superheating means.

In many power plants it is customary to utilize high temperature superheated steam in the high pressure stages of a'steam turbine forfdriving generators or the like and, for the purpose of maintaining the thermal emciency of the heat cycle, the exhaust steam from the primary stages of the turbine is frequently passed through a so-called reheater, which is a form of superheater, for the purpose of raising the temperature of the exhaust steam before admitting it to the secondary stages of the turbine. Where the superheater and reheater are located in the same boiler which also generates the steam, it is difficult to maintain uniform steam temperatures at all loads for both the primary superheater that supplies the high pressure turbine and for the reheater that supplies the-low pressure stages of the turbine. Although the temperature of gasespassing from the furnace over the superheater and reheater may be regulated inaccordance with variations in load the effect of the gases on the reheater and superheater is not necessarily proportional and uniform at the various loads because it is practically impossible to design two pieces of heat transfer apparatus so exactly that the desired results can be predicated and attained at varying loads.

An object of the present invention is to overcome these disadvantages by regulating the temperature of gases from the furnace in accordance with fluctuations in the outlet temperature of steam fromthe reheater and maintaining a desired primary stage steam temperature without further regulation of the temperature of gases flowing over the'reheater and then the superheater. The invention will be best understood upon consideration of the following detailed description of the illustrative embodiment thereof shown in the accompanying drawing in which the single figure is a diagrammatic elevational view of a water tube boiler having a primary superheater and steam reheater and in which the steam temperatures are regulated in accordance with the invention.

The boiler illustrated in the drawings includes a furnace [0 having its walls lined with water cooled tubes 12 connecting with the steam and water drum [4. The bottom of the furnace has two opposed sidessloping toward a throat I5 and faced with water cooled tubes l3. The two than." (01. 122-479) remaining side walls also are lined with water cooled tubes 12 extending downwardly to the throat Hi. The bottom and side walls above the throat thus form a water cooled hopper into which ash particles, gravitating from the fuel burning within the furnace, fall and, while falling, cool to a state in which they will not adhere to the sides of the hopper. Vertically adjustable burners l6 are arranged somewhat above the hopper bottom, and, preferably at the corners so as to project their streams of fuel and air into the furnace in directions tangential to the surface of an imaginary cylinder located vertically in the furnace, preferably in its center. Each set of burners shown discharges three fuel streams into the furnace. The rate of fuel firing is varied with changes in load but is not altered when regulating steam temperature at a particular load. The arrangement described above for the controlof the outlet temperature of gases from the furnace It is the subject of Patent No. 2,363,875 issued on November 28, 1944, to Henry Kreisinger and Virginius Z. Caracristi.

In operation'when all of the burners are adjustedto project the fuel and air streams in a substantially horizontal direction tangent to an imaginary vertical cylinder located centrally of the furnace, the burning fuel streams issuing from the four sets of corner located burners impinge upon one another resulting in a turbulent, rotating mixture with a consequent rapid rate of combustion. This rapid combustion produces a very high temperature in a zone A at the level of the burners near the bottom of the furnace so that there is a high rate of heat transfer from the flames to the water cooled wall tubes l2 in this zone both by radiation and by convection. Leaving this zone the hot products of combustion pass upwardly and continue giving off heatto the water cooled walls thereabove and exit from the furnace at a relatively low temperature. Greater heat absorption from the gases and a lower gas exit temperature may be achieved by depressing the burners below the horizontal so that combustion takes place in the zone D. When all of the burners are directed upwardly the streams of fuel are still tangent to an imaginary vertical cylinder in the furnace but the impingement of the burning fuel streams upon one' another is much less than when the burners are horizontally directed as above described. This results in but relatively little mixing and turbulence and consequent slower or delayed combustion. This delayed combustion occurs in :a zone B, that is located higher in the furnace than the zone A, and extends well into the upper part of the furnace. Consequently, the temperature of the products of combustion leaving the furnace is relatively high because of the shorter path through which they pass from the combustion zone to the outlet of the furnace and because the heat transfer from the flames to the water cooled walls at the bottom of the furnace is substantially less than with" horizontally directed streams. The change in the direction of the fuel streams from the horizontal to an upward inclination results in a rise in the temperature of the gases leaving. the furnace which has been found adaquate to change the rate of heat absorption by the reheaterflflrand.

the primary and secondarysuperheaters 'iI,J22 so that the steam temperature may be raised to that required for satisfactory operation over a greater range of ratings of the steam generating unit than heretofore attainable.

In addition to the primary superheater 2i and secondary superheaterlt the presenting vention utilizes a reheater, 2i Th,e,,high ternperature steam from the primary superheater 21 is taken from itsoutlet header 23 andionjts way to the inlet header 250i the secondary superheater 272 passesthrougha ,spray-type desuperheater 25 into which ,waterisinjected by regulation of a valve 25 ,subject'to fluctuations in the temperature of superheated steam;-in the secondary superheater outlet 21 ,as indicated at 23., From the outlet 2? of thesecondarysupe heater 22 the steam passes through aconduit to the highv pressurestages-of the steamturbine, not shown. The exhaust steam from thislstage of this turbine is returned through a conduit 32 to the inlet header 33 of the reheater Fill and after the steam is reheated, it is, directed through a conduit EA to the secondary; stage of theturbine. The final temperature-of stearnfrtmithe reheater 2c is regulated by tiltingtheburners l3 upwardly as the temperature of thezreheated steam falls or downwardly as this temperature becomes excessive, The regulation of the gas temperature at the outlet of thefurnace iii may be controlled automatically by providing motors 36 to tilt the burners it and-subjecting the operation of these motors to the outlet tem perature of steam from the reheaterr through a thermostat, 3B.

In order to obtain uniform; heat fromthereheater as over a wide range of capacity-the fuel burners for the furnace ill aretiltedupwardly or downwardly in responseto;decreases;or-increases in the steam temperature soas to vary the temperature of gas from the furnace in passing over the reheater, in such manner astomaintain a constant outlet reheat temperature. The temperature of steam for thehigh pressure stages of the turbine is maintained by, providing slightly more surface in the primary super heater than is required andthen controlling: the temperature bypassing the steam fromtheqprimary superheater 2! through the'desuperheater 25 to which water is admitted on fluctuation of outlet temperature from the secondarysuperheater 22. The primary superheater 2% need be over-surfaced only slightly becausethe telnperature of the gases that pass overit is being controlled for the reheater which atthegsame time effects a major controlforrtheqprimary superheater. For example, at half ratingabotut half of the maximum amount'of fuel thatamay be burned in the furnace -is-delivered;to the furnace near-the bottom and-burnedthere-in zoneA; the products of combustion likewise are iii) ,by a spray desuperheater because about half and dwell within the furnace and are exposed to the cooled walls about twice as long as at full rating. The exit temperature of the gases may then be too low for reheating the steam to the temperature desired. By raising the zone of combustion within the furnace and delaying combustion by decreasing turbulence, the dwell of the products of combustion within the furnace is reduced as is thearea of cooled furnace walls exposed to the gases.

It is deemed preferable not to control the primarysteam temperature by tilting the burners I6 and then to regulate the reheat temperature whatever wateriis used for controlling steam temperature should pass through the complete turbine cycle so; as to avoidiimpairing the work done in the several stages of the turbine. If an excessive reheattemperature were to be decreased by a water spray, an additional amount of steam would be produced which would then pass through the low pressure stage of the turbine without having passed through the highpressure section with the result that there would vbe a degradation of heat from the thermo-dynamic standpoint. I

What I, claim is:

1. The,methodofregulating temperature of steam from a superheater'andassociated reheater handling the same steam and absorbing their heat mainly by convection from gases leaving an outlet at one end of the same vertical water cooled tube furnace comprising; introducing. fuel and air from a plurality of points into a part of the furnace remote from the furnace outlet in such directions as to create a turbulent gas mass burning in a zone remote from the furnace outlet to transmit heat to the water tubes therein; maintaining, for a given load, a substantiallyuniformrate offuel firing; desuperheating the superheated steam from the superheater in, proportion tovariations of its temperature from a predeterminedvalue for maintaining he superheated steam temperature at said value; and altering in response to variations inreheat steam temperature 7 from a determined value the angle .to the horizontal of the'direction of the streams oifuel andair so as to move the location of the combustionzcne alongthe furnace axis further from or closerto the furnace outlet as the reheated steam temperature respectively increases or, decreases to thereby alter the extent to'which gases are cooled before-reaching the reheater and the superheater.

2-. The methodof,regulating-temperature of steam from a superheater and, associated reheater handling the samesteam and absorbing their heat mainly .byconvection from gasesleavinganyoutlet at oneend of thesaine water, cooled tube-furnace comprising; introducing fuel-and air from a plurality of points intoa; part ofthe furnace remote from the furnace outlet .in such directions as to create a turbulent gas mass burning in a zone remotefrom the furnace outlet to transmit heat to the watertubes therein; maintaining, for a given load, a uniform rate of fuel firing; injecting water into the super heated steam in proportiontovariations of'its temperature from a predetermined value -for maintaining the superheated steam temperature at said value; and'altering' in response-to variations in reheat steam temperature from-'adeterminedvalue I the angle tothe horizontal of the directionof the streams-:of fuel and aii soas .to

- move the location'of the combustion-zone ;along the furnace axis closer or further away from its outlet as the reheated steam temperature respectively increases or decreases to thereby alter the extent to which gases are cooled before reaching the reheater and the superheater.

3. The method of regulating temperature of steam from a superheater and associated reheater handling the same steam and absorbing their heat mainly by convection from gases leaving an outlet at one end of the same vertical water cooled tube furnace comprising; introducing fuel and air into the furnace from a multiplicity of points remote from the furnace outlet and in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating gas mass in a zone located in a portion of the furnace remote from its outlet to transmit heat to the water tubes therein; maintaining, for a given load, a substantially uniform rate of fuel firing; and, as the reheated steam temperature increases, decreasing the vertical angle of the directions of fuel introduction relatively to the furnace walls to thereby increase the extent to which gases are cooled before reaching the superheater and reheater; and injecting Water into the superheated steam in proportion to fluctuations of its temperature from a predetermined value.

4. The method of regulating temperature of steam from a superheater and associated reheater handling the same steam and absorbing their heat mainly by convection from gases leaving the outlet of the same water tube cooled furnace comprising; introducing fuel and air into the furnace for creating a gas mass to radiate heat to the water tubes thereof maintaining, for a given load, a substantially uniform rate of fuel firing; passing the gases from the furnace over the reheater and reheating steam passing therethrough, directing the gases subsequent to their passage over the reheater into contact with superheating elements of a capacity capable of heating the steam to a temperature above a predetermined value desired at other loads than the maximum designed load; and as the reheated steam temperature increases, increasing the amount of cooling of the gases of combustion in imparting heat to said water tubes prior to discharge from the furnace so as to lower the temperature of gases flowing over the heater and reheater; and injecting water into the superheated steam in proportion to fluctuation of its tempera ture from said predetermined value to reduce said temperature to said value at loads other than the maximum designed load.

WILBUR H. ARNIACOST.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,732,009 Gilbert Oct. 15, 1929 1,773,088 Armacost Aug. 19, 1930 1,886,076 Abendroth et a1 Nov. 1, 1932 1,915,052 Dettenborn June 20, 1933 1,949,866 Heiet Mar. 6, 1934 1,972,356 Pfleiderer Sept. 4, 1934 2,092,299 Black et al Sept. 7, 1937 2,196,377 Bailey Apr, 9, 1940 2,271,886 Juzi Feb. 3, 1942 2,312,375 Whitney Mar. 2, 1943 2,319,223 Frisch May 18, 1943 2,363,875 Kreisinger et a1 Nov. 28, 1944 2,421,761 Rowand et a1 June 10, 1947 FOREIGN PATENTS Number Country Date 482,901 Great Britain Apr. 7, 1938 516,070 Great Britain Dec. 21, 1939 

